1. Field of the Invention
The present invention relates to a developer supplying member, and more particularly to a developer supplying member such as a developing sleeve for use in the developing device of a copying apparatus.
2. Description of the Prior Art
In general, electrophotographic copying methods employ developing processes of which the Carlson process is representative. Electrostatic latent images are formed by charging a photoconductive photosensitive member and exposing it to light. Thereafter, a bicomponent developer comprising a toner and a carrier, or a monocomponent developer comprising toner alone, is retained on the surface of a rotatable developer supplying member and comes into contact with the aforesaid electrostatic latent image, thereby developing said latent image as a toner image. One of the well-known disadvantages inherent to these conventional developing methods is the problem of image fog. Fog appears due to electrostatic toner adhesion induced by a residual charge which is incurred during the developing process when the previous charge is not completely removed from the exposed porion of the photosensitive member. Developing then results in a generally soiled copy image. The application of a developing bias voltage is commonly used to eliminate this disadvantage.
The developing bias voltage method involves the application, with each development cycle, of a d.c. voltage or a d.c. voltage superposed on an a.c. voltage to an electrode provided on the developer supplying member, said voltage being of the same polarity as the residual potential of the exposed portion of the photosensitive member. At the same time, either hhe photosensitive member which acts as the opposed electrode or an electrically conductive substrate which is the single component most essential to the structure of the photosensitive member is grounded, thereby forming an electric field therebetween. The repulsion between the charge polarity of the exposed portion of the photosensitive member and the polarity of the applied bias voltage is used to prevent adhesion of the toner to the aforesaid exposed portions. Although the above described method relates to regular developing processes wherein the developing action occurs via an electrostatic image formed on the photosensitive member and an oppositely charged toner, a bias voltage may also be applied by the same method in cases of reverse developing which use a latent image and similarly charged toner, despite differences in polarity or potential. This developing bias application method effectively prevents the appearance of copy image fog, but conversely gives rise to new problems.
For example, when an electrical leak occurs between the electrode on the developer supplying member side and the electrode on the photosensitive member side, fog may occur due to a reduction in the developing bias potential. Many factors can cause this type of electrical leak, for example, in bicomponent developing, magnetic particles normally used as developer carriers may penetrate the layers of the photosensitive member and reach the electrically conductive substrate. Layer penetration of this type may occur when the photosensitive member has a low hardness, and is a particular problem for organic photosensitive members due to their low hardness, although they have seen remarkable recent developments. Furthermore, since pinhole-like defects are sometimes present in the photosensitive layer of photosensitive members, this problem may occur in bicomponent developing when the magnetic particles normally used as carriers and the electrically conductive substrate of the photosensitive member make contact, or in monocomponent developing, when the electrode on the developer supplying member and the electrically conductive substrate of the photosensitive member make contact. The reduction of so-called bias potential based on this type of developing in actual copying machines leads to long narrow bands of fog on the copy image and a marked reduction in copy image quality despite some developing in the defective section which is induced by the leak itself because not only the portion where the leak occurs but rather the entire portion where the developer and photosensitive member make contact is affected.
On the other hand, in monocomponent developing, the toner must be charged prior to developing with a specified polarity in accordance with the polarity of the electrostatic latent image and the type cf developing, either standard or reverse. For example, methods using gapped triboelectrically charging blades, methods of toner charging via a corona charger, and methods of imparting a charge to the toner by having the toner make contact with a metal plate which has a bias voltage applied thereto are all considered in toner and triboelectrically charging order. However, the triboelectrically charging blade method is either unsuitable for high speed developing because of inadequate toner charging, cr a plurality of triboelectrically charging blades must be provided so as to adequately charge the toner. The methods employing a corona charger cause dispersal of the toner due to the action of the electric field generated during corona charging, and do not adequately charge to the interior of the toner layer. Furthermore, methods whereby a charge is imparted by a metal plate do not present the aforesaid disadvantages to any great degree. When a difference in potential is formed between the developer supplying member and the metal plate by the conductivity of said member so as to charge the toner as it passes therebetween, the electrode of the developer supplying member is overly effective during developing, thereby causing an inadequate edge effect. In developing which uses a single compcnent toner, an extremely large electrode effect arises in the vicinity just below the toner layer where the electrostatic latent image and the surface of the developer supplying member are in proximity because the thickness of the toner layer on the developer supplying member is at most 50 .mu.m. Accordingly, gradients are poor, as is the reproducibility of low density original documents.
In order to eliminate these disadvantages, a high resistance layer or a layer with a high degree of hardness is provided as a covering on the developer supplying member. That is, the photcsensitive member and develooer supplying member have a low-resistance layer disposed therebetween so as to prevent the aforesaid reduction of bias pctential, inadequate charging or excessive electrode effect.
For example, U.S. Pat. No. 4,086,873 discloses a developing method for electrophotographic copying processes wherein the surface of an endless member is provided with a high-resistance layer via an aluminum anodizing process, or a silicon resin, urea resin, melamine resin, polyvinyl butyral resin and the like as a surface protective layer. Further, Unexamined Japanese Patent Publication Sho No. 55-46768 discloses an electrostatic latent image developing apparatus with a developer supplying member provided with a surface layer of silicon rubber, neoprene rubber, nitrile rubber and the like, of about 5 mm in thickness and which provides a volumetric specific resistivity of 10.sup.8 ohm-cm to 10.sup.15 ohm-cm.
Technology for providing surface covering layers for developer supplying members in order to eliminate the aforesaid deterioration of copy image quality induced by reduction of bias potential, inadequate charging or excessive electrode effect, require that said cover layer be of suitably high resistance. Further, the cover layer must employ a material with low fusability vis-a-vis the developer and a high degree of hardness so as to avoid damage contact with other members or frequent developer soiling during actual use in a copying machine. The cover layer must be without resistance irregularities over the entire region of said layer, and must have a homogeneous, uniform thickness to maintain a so-called developer gap of specific measurement between said cover layer and the photosensitive member. A developer supplying member having with such a cover layer must also provide adequate developer transportability. However, conventional examples cannot necessarily be said to adequately meet these performance requirements, therefore cover layer materials of higher performance characteristics are needed.